You can place a constant value like "1.3f" in the value params, but many times you
would want to pass in external information to use per request. For instance, maybe
you want to rank things differently if the search came from a mobile device, or maybe
you want to use your external query intent system as a feature.
In the rerank request you can pass in rq={... efi.userFromMobile=1}, and the above
feature will return 1 for all the docs for that request. If required is set to true,
the request will return an error since you failed to pass in the efi, otherwise if will
just skip the feature and use a default value of 0 instead.